Miocene radiolarian assemblages were collected from Maria Madre and Maria Cleofas islands and five localities along Baja California (Tortugas, Bahia de Asuncion, El Cien, Cabo San Lucas-La Paz, and San Felipe). Most of these sequences are composed of diatomaceous and/or diatomaceous shales or very fine sands.

Stratigraphic correlation with previous work on the Monterey Formation, Experimental Mohole, and DSDP Legs 5, 7, 18, 32, and 66 indicate that the radiolarian zones Diartus petterssoni, Didymocyrtis antepenultima, Didymocyrtis penultima, and Stichocorys peregrina are present in these newly worked sections.

The oldest dates obtained were from the Tortugas Formation (middle Miocene); the youngest was uppermost Miocene from the section at San Felipe.

Paleoenvironmental interpretations were based on quantitative analyses on warm, cold, deep, and intermediate water forms, as well as abundances of diatoms and silicoflagellates. The Tortugas Formation (middle to upper Miocene) represents a slope biofacies characterized by relatively high percentages of cold and intermediate water forms, and the absence of deepwater radiolarians. Diatomaceous layers increase upsection and show an increase in radiolarian abundance and diversity (especially of cold water forms and silicoflagellates). These conditions reflect an enhancement of upwelling areas and low oxygen depositional facies.

Previous radiolarian and diatom studies in Maria Madre and Bahia de Asuncion as well as Maria Cleofas, indicate that these environments of deposition are analogous to the Monterey Formation. Radiolarian faunas from these three sections are correlated to the Didymocyrtis antepenultima Zone and reflect an intensification of upwelling, perhaps as a result of the buildup of the Antarctic ice cap.

The "El Cien" section and the section collected between La Paz and Cabo San Lucas are representative of the Didymocyrtis penultima Zone. The "El Cien" sequence is underlain by pillow basalts that may represent the contact between the East Pacific Rise and North America. Radiolarian faunas in both sections represent a mixture of warm and cold water forms with considerable dilution by terrigeneous sediments.

San Felipe, the only section located on the gulf side, contains a unique radiolarian/fauna. This unique fauna may have been the result of "basin isolation" during the opening of the Gulf of California.

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